The overarching objective of this application is to achieve a detailed understanding of the first cultivated TM7 phylotype from the human oral cavity and explore its pathogenic potential. Since the initiation of the Human Microbiome Project, a tremendous diversity of bacteria has been discovered in the human body, many of which have no cultured representatives. Studying these supposedly uncultivable bacteria presents a major challenge to the field. Members of the applicant division TM7 phylum are the most prevalent yet elusive of these bacteria. TM7 has been found in various human body sites and environmental samples, and also been linked to human mucosal diseases, particularly severe and ulcerative periodontitis, vaginosis, and inflammatory bowel disease. Therefore, the impact of this study is twofold: it will increase our knowledge of uncultivable bacteria and will illuminate the pathogenic potential of TM7 members. Using novel cultivation methods, our lab recently cultured the first TM7 member (TM7x) attached to the surface of a previously uncultivated Actinomyces odontolyticus strain (XH001) from the oral cavity. TM7x was not able to grow on its own, suggesting that the presence of XH001 is crucial to TM7x's survival. In contrast, XH001 had accelerated cell death and sporulation in presence of TM7x. This type of epibiotic parasite interaction is completely novel and has never been observed in oral microbiota. Furthermore, TM7x, when associated with XH001, can prevent the induction of TNFa gene expression by macrophages, suggesting its possible role in modulating the immune response. Yet many questions remain unanswered: Why does TM7x need XH001? Does TM7x adhere to the surfaces of other oral bacteria? What are their individual and combined pathogenic potentials? Based on our initial findings, I hypothesize that physical and/or metabolic interactions between TM7x and XH001 are crucial for the growth of TM7x and have a great impact on their pathogenicity. Uncovering this interaction at the molecular level will elucidate th impact of TM7x on human health and disease. To explore these hypotheses, I will carry out four interconnected but independent aims: (1) I will first thoroughly characterize the morphological and physiological changes of TM7x and XH001 during different stages of co-culture growth. (2) I will next analyze the global gene expression of each growth stage to uncover the underlying genetics of the observed morphological changes. (3) I will manipulate gene expression of TM7x or XH001 to further evaluate the interaction between TM7x and XH001. (4) Once I understand the interaction between TM7x and XH001, I will apply this knowledge to study the pathogenic potential of TM7x in association with XH001.